Light attenuation parameters for waste stabilisation ponds.

Effective modelling of shallow water ecosystems, including waste stabilisation ponds, is strongly dependent on the availability of good estimates of the light attenuation coefficient k (m(-1)). Experimental data is presented on its determination using purpose-built laboratory apparatus with a near-parallel halogen light source and an array of photodiodes allowing measurements of irradiance at different depths. The equipment was used to compare k values from 4 different pure cultures, and mixed cultures of algae taken from a pilot-scale WSP. Laboratory values were compared with in situ measurements in the pond. At concentrations above 50 mg l(-1) the relationship between k and suspended solids is non-linear; k also varied with depth. This could be modelled by a single equation, suggesting similarity of response in different cultures. At shallow depths and low suspended solids concentrations k values are variable and hard to measure reliably. The results highlight the need to standardise on a method for the measurement and reporting of k values if these are to be widely applicable in the development of pond models.